U.S. patent application Ser. No. 822,457, entitled "Process And Apparatus For Analyzing A Gaseous Mixture And A Visible Emission Spectra Generator Therefor," filed Jan. 17, 1986, has a common assignee with present invention and is directed to a process and apparatus for measuring trace amounts of a gas in a gaseous mixture, and to a visible emission spectra generator for gaseous mixtures. The apparatus comprises a chamber for receiving the gas to be analyzed and having electrodes positioned externally about the chamber and longitudinally disposed with respect to one another. An r. f. energy source connected to the electrodes causes an r. f. glow discharge reaction and resulting r. f. current there between. This generates a visible emission spectra of the gaseous mixture to be analyzed. The chamber is a glass tube with spaced apart electrodes disposed about the outside thereof. A photodetector is disposed proximate the chamber and detects light generated or a visible emission spectra. The process and apparatus also include a data processing device for evaluating spectral signals received by the photo detector device and suitable display and/or recording assemblies. The total current (Itot) drawn during an r. f. glow discharge reaction comprises a first component (Il) which flows through the r. f. glow discharge reaction and a second component (Is) which flows through stray capacitance between the electrodes. It is necessary to keep Il essentially constant to obtain accurate data. Keeping Itot constant does not necessarily keep Il constant.
It is desirable in some applications to provide apparatus for maintaining essentially constant current through an r. f. glow discharge reaction housed in a chamber in the presence of parasitic (stray) capacitance between electrodes of the chamber which also draws current.